When you say that the module lets you know how many items are present in your warehouse, do you mean a) it gives you a report of all the stock levels recorded by uc_inventory, or b) it pulls info from an external database that records what is actually in your physical warehouse?

Since a) is already accomplished by uc_inventory, I'm guessing the module does something like b).

Actually, this module adds a "inventory status report" to the reports menu. This report shows a list of all the products that you have setup to be tracked the the simple stock manager module, and how many are currently in stock. It basically provides the report functionality to Inventory API that the integrated stock module has.